[cfe-dev] Trying to figure out where a front-end optimization occurs

[Friedman, Eli via cfe-dev]

On 1/23/2017 4:24 PM, bharathi seshadri via cfe-dev wrote:
> I'm trying to understand where, in the front-end, the if block in the
> example shown below is optimized away (with optimization level -O1 and
> above).
>
> $ cat test.c
> void foo(int x);
> int main() {
> int x = 0;
> if (x)
>   foo(x);
> return 0;
> }
> $clang -emit-llvm -O2 -S test.c
>
>
> I tried to figure this out by debugging clang using gdb, but without
> success. I see that in CodeGenFunction::EmitIfStmt, the call to
> ConstantFoldsToSimpleInteger(), which appears to help decide removal
> of the dead arm of the if/else, returns false in this case and the
> code proceeds to emit the IF block. I see that
> CodeGenFunction::EmitCall is also being invoked for foo(). But the
> final output (test.ll) shows that the if block is optimized away and
> there is no call to foo().
>
> Any pointers would be greatly appreciated!
>

echo "int g(); void f() { int x = 0; if (x) g(); }" | clang -x c - -o - 
-S -mllvm -print-after-all -O2 |& less

That shows the IR after each step of the optimizer.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
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